Added erfcinv

[peppelauro]

Add function erfcinv (Inverse Complementary Error Function)

[dougthor42]

Wonderful, thanks! Overall it looks great - very clean. A couple in-line comments.

Please do the following:

1. Add unit tests to pyerf/tests/test_pyerf.py for:
   1. known values (especially at the extremes)
   2. exceptions raised
   3. comparison to numpy/scipy results
2. Remove trailing whitespace from lines.

You can ignore the failing build - that's pip 18 dropping support for Py3.3. I'll fix that in another MR.